That is a likely response. But that will push authors who disagree with that to other publishers.

I've noticed a number of technical books developed in an open manner (though usually not under the GPL) that then sold well. There are also a lot of good people around software these days who strongly believe in the ideals of free and/or open source software. Therefore I suspect that insisting on strict copyright control will lose more sales than it gains. But people will make mistakes as they learn this.